A mobile wireless communication system includes a mobile station and a base station. The mobile station controls transmission power that is used when transmitting data in an up link (UL) (hereinafter referred to simply as “UL transmission power”). The base station receives data transmitted from the mobile station, and detects a reception level. Upon determining that the detected reception level is low, the base station transmits a command to increase the UL transmission power to the mobile station by using a down control channel. In contrast, upon determining that the detected reception level is high, the base station transmits a command to decrease the UL transmission power to the mobile station by using the down control channel. A transmit power control (TPC) value is contained in such a control command for UL transmission power. The mobile station receives a command transmitted from the base station. The command instructs the mobile station to control the UL transmission power. The mobile station then extracts a TPC value from the received command that instructs the mobile station to control the UL transmission power. Subsequently, the mobile station controls the value of the UL transmission power using the extracted TPC value. Here, in wireless communication systems, data is handled on a frame-by-frame basis, which is a time basis. At the time of transmitting and receiving data, subframes obtained by dividing a single frame are used for transmitting and receiving data. Control of the UL transmission power value by the mobile station is performed for each subframe.
In conventional UL transmission power control in wireless communication systems, upon receiving a control command from a base station, a mobile station performs control in accordance with the specified TPC value. Unfortunately, under deteriorated conditions of the propagation path from the base station, some of control commands for UL transmission power transmitted from the base station are erroneous, and these erroneous commands are transmitted from the base station in some cases. In these cases, there has been a possibility that although transmission power is to be decreased, the mobile station receives a control command for UL transmission power that instructs the mobile station to increase the transmission power. Accordingly, transmission power control that demands excessive transmission power would be performed, resulting in increased power consumption of the mobile station and interference with signals of other mobile stations on the base station side.
To address the above issues, there has been proposed a conventional technology in which, upon receiving commands to excessively increase transmission power from a base station, a mobile station performs mask processing that disregards control commands for UL transmission power from the base station. There has been proposed another conventional technology in which, in the case of not using a channel for exclusive use of packets, transmission of a control command using a transmission power control channel is not performed, so that wasteful consumption of wireless resources is avoided.
Japanese Laid-open Patent Publication No. 2006-186757 and International Publication Pamphlet No. WO 2004/027676 are examples of the related art.
Unfortunately, in the latter conventional technology, in which transmission of a control command using a transmission power control channel is stopped, it is not determined whether an instruction for increasing transmission power will result in excessive transmission power. This conventional technology therefore faces difficulty in imposing a limitation on an excessive increase in transmission power. In the former conventional technology, which disregards control commands from a base station when commands for excessively increasing transmission power are received, the distance between the base station and a mobile station is not taken into account. In reality, desirable UL transmission power varies as a function of the distance between a base station and a mobile station. In this conventional technology, unfortunately, there has been a difficulty in that whether transmission power is excessive is determined in accordance with the control distance, and thus there has been a difficulty in that UL transmission power is controlled appropriately in accordance with the distance between a base station and a mobile station.